1. Field of the Invention
This invention relates generally to the reduction of noise in signals having an amplitude that varies over time including, but not limited to, signals generated by impacts, transients, speech, RF (radio frequency) sources and the like, and that are subject to external and/or intrinsic noise. More particularly, this invention relates to a method and apparatus for reducing the noise associated with acoustic sensor outputs, including the output signals of seismic sensors, such as those employed in the measurement of underground pipeline noise. The invention reduces non-white noise and correlated noise, as well as white noise, from the output of sensors placed on pipelines for reasons including monitoring for pipeline strikes or impacts by excavation equipment, falling rocks, etc., and monitoring of pipeline movement, such as movement caused by earthquakes.
2. Description of Related Art
Underground pipelines are widely used to transport a variety of fluids, including oil, natural gas, water, etc., from one place to another. Such underground systems are subject to damage from a variety of sources, both naturally occurring and man-made. For example, subsidence of the soil, local construction projects, seismic activity, and weather can all lead to defects and anomalies in the pipeline. Also, harsh environments can cause pipelines to move gradually over time, leading to defects, cracks, leaks, bumps, and other anomalies, within the interior of the pipeline.
Continuous monitoring of long pipelines, whether for seismic events or impacts occurring during excavation in proximity to the pipeline, is not a simple task. Damage to pipelines can be detected in a variety of ways including detection of the substance that escapes from the pipeline as the result of the damage, pressure drops in the pipeline, and impacts on the pipeline.
There are several systems and methods known to those skilled in the art for continuously monitoring the condition of underground pipelines. Acoustic monitoring of an underground pipeline may be carried out by a variety of acoustic sensors/detectors, such as geophones, accelerometers and the like. One problem with the use of acoustic means for monitoring underground pipelines is noise, both background and sensor noise, in the output signal from these means, which noise may partially or possibly completely mask the signal of interest, thereby precluding detection of the pipeline condition. For example, while sounds associated with impacts on a pipeline can be transmitted through the pipeline and detected at substantial distances from the point of contact via such highly sensitive acoustic sensors, the high sensitivity of such sensors can produce or result in a significant number of false calls arising from sources such as passing vehicles and weather conditions such as thunder. Accordingly, to better enable the use of acoustic means for monitoring underground pipelines, it is desirable that the noise in the output signals from the acoustic sensors/detectors be substantially reduced or, if possible, entirely eliminated.